Nearly 300 people are missing and at least four are confirmed dead after a ferry carrying 462 people, many of them students, sank in cold waters off South Korea’s southern coast Wednesday.

There were fears of a big jump in the number of deaths, as dozens of boats, helicopters and divers scrambled to rescue passengers who had been on the ferry travelling to the southern island of Jeju. Those rescue efforts will continue overnight. One rescued passenger said he believed that many people had been trapped inside the ferry when it sank.

The confirmed fatalities are a female crew member, 22, and a male student.

The ferry, identified as the Sewol, was sailing to the southern island of Jeju when it sent a distress call as it began leaning to one side. The passengers include more than 300 students from Danwon High School in Ansan, near Seoul, who were on a school trip.